Ok, I have just started the chapter Gases and the Ideal Gas Law.

I am to sketch the graph for P (pressure) against T (temperature in Kelvin) whilst the number of moles and volume are kept constant and I got a graph similar to that of an y = x graph.

Now, I am to graph another one for the case of a gas which decompose on heating, N2O4, giving 2 moles of NO2. I can't seem to imagine such a situation.

What I've come up till now is that upon heating, decomposition occurs, hence and increase in pressure at first. However, heat will certainly be absorbed in the decomposition process. If so, there would be a sudden sharp increase in pressure and then, the gradient of the graph decreases to become once more linear. Am I right? I'll reply in about 30 min. (have other work to do... )

:)

I have to sketch the graph for P (pressure) against T (temperature in Kelvin) whilst the number of moles and volume are kept constant and I got a graph similar to that of an y = x graph.


Ideal gas law:

PV=nRT

Since you don't know n, the number of moles, you can't do any explicit calculations. Only P and T vary.

P = \frac {nRT}{V} = \frac kT and P is proportional to T. So, you have a line through the origin (T=0 on an absolute temperature scale) of slope nR/V



Now, I am to graph another one for the case of a gas which decompose on heating, N2O4, giving 2 moles of NO2. I can't seem to imagine such a situation.

What I've come up till now is that upon heating, decomposition occurs, hence and increase in pressure at first. However, heat will certainly be absorbed in the decomposition process. If so, there would be a sudden sharp increase in pressure and then, the gradient of the graph decreases to become once more linear. Am I right? I'll reply in about 30 min. (have other work to do... )



If NO2 and N2O4 are considered ideal gases (they probably aren't), you are increasing T, and at the rate of decomposition, increasing n. V is constant so P will increase.

P=nT (\frac RV) = knT

n only increases until all of the N2O4 is consumed. So, I think you're right. After all of the N2O4 is consumed, P will be proportional to T, as in the first problem.

I was just wondering why we had to plot PV against P? I know we get a straight horizontal line, but I can't seem to see it... :(

P against V, OK, the graph is like y = 1/x
P against 1/V, OK, the graph is like y = x

taking PV = k for the ease of graphing and 'juggling' with the equation.

I am to sketch the graph for P (pressure) against T (temperature in Kelvin) whilst the number of moles and volume are kept constant and I got a graph similar to that of an y = x graph.


Did you say PV against V? I can't see that. I'm not sure why you would plot PV against P. That would be equivalent to plotting P against V or vice-versa.

Oh, sorry that was another separate question, but since it is in the same topic...

PV against P gives a straight line

However, P against V gives a graph like that of y = k/x where k is a constant.

I'll rephrase my question, how do I know what graph to get if I'm required to plot PV against P.

PV against P gives a straight line

However, P against V gives a graph like that of y = k/x where k is a constant.

I'll rephrase my question, how do I know what graph to get if I'm required to plot PV against P
.


Hmmm.

PV=nRT

At constant T and n, PV is always a constant so PV is a constant. If T changes, PV will be proportional to T. P will also be proportional to T if V is constant. You'll end up with a straight line.

But, that's just for ideal gases. If PV isn't constant, you'll end up with a curved line -- deviations from the ideal gas law.

Let me find something:

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch4/deviation5.html

http://library.thinkquest.org/C006669/data/Chem/gases/vanderwaals.html

Ah! Those links helped! I'm going to go through them... There was a question about ammonia which would have followed, but I saw it in the link. Thanks! :)

Ok, I understood it! Thanks again Perito! :)